The rims of turbine wheels are often provided with axially spaced, annularly extending fingers that define dovetails. These dovetails receive generally complementary shaped finger dovetails on buckets that are to be secured to the wheel. A number of pinholes may be aligned axially through the bucket fingers and the wheel fingers along the margin of the wheel. The pins may be axially inserted through these pinholes to secure the buckets to the wheel.
Over time and extended use, radial loading on the pins and the presence of a corrosive environment in the turbine may cause stress corrosion cracks to develop about the pinholes. In addition, stress corrosion cracking can occur on the wheel finger ledges where the wheel fingers and bucket fingers fit together. These cracks appear to initiate mid-way between columns of pinholes on the wheel fingers. At these locations, the fingers of adjacent buckets butt together and form a crevice extending between neighboring wheel fingers. The cracks may grow circumferentially along the wheel finger ledges toward the nearest pinholes while also growing axially through the finger. These cracks may lead to the failure of a finger and potential damage to the turbine as a whole.
As a result of this possible damage, periodic inspections of the wheel and the bucket finger dovetails are indicated. These inspections generally involve driving the existing pins out of the pinholes to allow the buckets to be removed from the wheel. A florescent magnetic particle inspection of the finger surfaces or other types of inspections then may be performed. For example, the magnetic particles collect around any surface breaking cracks in the presence of an applied magnetic field. The inspector may then illuminate the area with a black light such that the magnetic particles fluoresce. Any cracks present in the finger then may be visually identified. This inspection method, however, requires extensive disassembly of the wheel and the buckets such that the method is labor intensive, time consuming, and hence, costly.
More recent improvements have led to inspecting the turbine wheel and bucket finger dovetails via a phased array ultrasonic probe inserted within a pinhole. Unlike the magnetic particle inspection, the buckets need not be removed for the ultrasonic inspection and only a fraction of the pins must be removed for probe insertion. The phased array probe is designed to produce an ultrasonic beam directed radially outward from the pinhole that is electronically rotated to inspect the surrounding finger material. A similar inspection may be performed by an ultrasonic probe in which the radially-directed beam produced by a single transducer element is rotated mechanically. This inspection method may detect cracks occurring at the adjacent pinholes and along the finger ledges. However, at the ledges the orientation of the cracks may prevent the ultrasonic beam from reflecting back to the probe. Specifically, cracks that occur on the finger ledges are generally located between adjacent pinholes and have an axial-circumferential orientation. The ultrasonic beam has an angle of incidence on these cracks that results in the beam being reflected away from the probe. Consequently, these cracks may not be identified by the same analysis method used to identify cracks occurring at the adjacent pinholes.
There is a desire, therefore, for an improved method of analyzing the ultrasonic inspection data from turbine wheel and bucket finger dovetails, particularly at the ledge region.